When loads are supplied via a mains-operated power supply, precautions are normally taken for the situation in which the power line system which drives the power supply fails briefly. As a rule, such a power line system is an AC power line system. Alternatively, it may also be a DC mains system.
The failure of the supplying power line system, or its voltage falling below a specific value, is, as a rule, identified and reported by special detection devices at the input of the power supply. These devices then, for example, connect a battery to the loads, using a relay or a semiconductor switch. As soon as the supplying mains system has reached a specific voltage value again, the switch is opened again. Furthermore, power supplies are frequently intended to be designed such that they provide an output DC voltage which is safely isolated from the supplying power line system at the input. This is the case, in particular, when the mains system is a 400 V or 230 V AC mains system. Here, it may be necessary for the power supply to emit a so-called SELV DC voltage in accordance with EN 60950 (EN: European standard) for example with a value of 24 V (SELV: Safety Extra Low Voltage).
In such a case, it is also necessary for the detection device for the voltage which is monitored on the power line system input side to satisfy the requirements for safe isolation. This is disadvantageous since the large air gaps and leakage paths required, inter alia, for this purpose necessitate an increased cost and space outlay, and correspondingly costly components.
A further disadvantage of direct detection of the supplying power line system voltage on the input side of the power supply is that, in the event of a failure of the power supply itself, the battery is not connected since the detection device in it does not identify a fault because the power line system voltage is present without any change. However, because of the high stress in terms of current, voltage and temperature, power supplies are among the particularly severely stressed devices and therefore, as a rule, fail earlier than most of the loads supplied by them. Such devices therefore do not identify, for example, failure of the drive of the primary transistors, the power supply transiently not running up, or any component or a connection in the primary or secondary circuit becoming open circuit. The battery is connected only in the particularly serious case in which the power supply fails as a result of a primary short-circuit and an upstream automatic protection device disconnects the supplying power line system as a consequence of this.
A further problem is that many loads on the DC side require high starting currents, for example on connection, for example incandescent lamps, DC/DC converters, capacitors, DC motors, switching contactors with a DC economy winding and the like. Some power supplies disconnect immediately in the event of an overload. This is disadvantageous since a sudden, complete interruption occurs in the supply to the load. Other power supplies have automatic current limiting, as a result of which the output voltage collapses in accordance with Ohm's Law U=R.sub.load .times.I.sub.current limiting. Here, the loads are supplied with an unacceptably low voltage in a disadvantageous manner for the time period of the voltage dip caused by high starting currents.
In conventional circuits, the return of the output DC voltage of the power supply can be identified, independently of the power line system input voltage by a diode which is connected in the output line, which carries the positive potential, between the power supply and the load. The load voltage, for example the 24 V output DC voltage of the power supply less the forward voltage of the diode V1 when the battery is disconnected or the battery voltage when the battery is connected, is present on the cathode of this diode. The actual output DC voltage of the power supply can be detected on the anode of the diode. If the power supply is not supplying any output voltage, then the voltage at the anode of the diode is virtually 0 volts. A control signal can be derived from this which maintains the connection of the battery. If the voltage at the anode rises above a predetermined value, the battery is disconnected from the loads again to prevent it from being unnecessarily discharged. In the steady state, the diode thus makes it possible to distinguish with certainty whether the DC voltage on the load is being provided by the power supply or the battery. The disadvantage associated with using such a diode is the occurrence of a voltage drop, caused by the output DC voltage, of about 1 volt, a high power loss, and an increased space requirement for a large heat sink.